Government interest in RFID animal identification developed in the late 1980's with the formation of the European Union and ensuing concerns about potentially uncontrolled transport of animals across international borders. In response, the International Standards Organization was chartered in the early 1990's with developing an international standard based on one or more of the existing RFID technologies, with the intent of identifying all livestock with RFID tags. A practicable and operable international identification system requires a standard RFID system, rather than a proliferation of the multiple mutually incompatible proprietary systems. After several years of research, investigation, and negotiation, ISO published Standards 11784 and 11785.
ISO Standard 11785 defines the technical principles for communications between interrogators (alternately referred to as “readers” or “scanners”) and two types of electronic passive identification transponders, and ISO 11784 defines the allocation of transponder memory bits for identification purposes. These transponders contain identification information stored in binary form, which is conveyed to the interrogator when a transponder is suitably activated by the interrogator. Additional technical details are provided in ISO Standards 11784 and 11785, the disclosure of which is incorporated into this disclosure by reference in its entirety.
Passive low frequency RFID interrogators and tags use operating principles that are well-know to those of ordinary skill in the art, and that are described in extensive detail in several seminal inventions, including U.S. Pat. No. 1,744,036 (Brard—1930), U.S. Pat. No. 3,299,424 (Vinding—1967), U.S. Pat. No. 3,713,146 (Cardullo—1973), and U.S. Pat. No. 5,053,774 (Schuermann—1991), and in textbooks such as “RFID Handbook” (Finkenzeller—1999).
As depicted in FIG. 1, the interrogator 100 includes electronic circuitry, which generates an activation signal (usually a single frequency unmodulated sinusoidal signal) using a signal source 101 and an amplifier 102 to drive a resonant antenna circuit 103. This activation signal manifests as a time-varying electromagnetic field, which couples with the ID tag 105 by means of the electromagnetic field's magnetic field component 104. The ID tag 105 converts this magnetic field into an electrical voltage and current, and uses this electrical power to activate its internal electronic circuitry. Using any of several possible modulation schemes, the ID tag conveys binary encoded information stored within it back to the interrogator via a magnetic field 104, where the detector and utilization circuit 106 converts this binary code into typically decimal, hexadecimal, or alphanumeric format tag data 107 in accordance with some prescribed application.
ISO Standard 11785 defines two types of transponder technologies, which are designated “full-duplex” (“FDX-B”) and “half-duplex” (“HDX”). In the described manners that follow, for HDX and FDX-B transponders, respectively, activation energy is transferred to the transponder from the interrogator, and identification code information is transferred to the interrogator from the transponder through the mutual coupling of a magnetic field.
The FDX-B transponder amplitude modulates the interrogator's activation signal with its binary identification code sequence. The interrogator detects this modulation and derives from it the FDX-B transponder's identification code. The term “full-duplex” derives from the FDX-B transponder's behavior wherein its identification code information is transmitted simultaneously during receipt of the activation signal from the interrogator.
In contrast, the HDX transponder uses the interrogator's activation signal to charge an internal capacitor (which functions as a very small rechargeable battery), and it uses this stored energy to activate a transmitter, which emits a frequency shift keyed (“FSK”) signal representative of the transponder's identification code. Specifically, the binary identification code information contained in the HDX tag is serially output such that the occurrence of a binary “1” results in an ISO HDX tag's radio signal being 124.2 KHz and a binary “0” results in the tag's radio signal being 134.2 KHz. The interrogator detects this FSK signal and derives from it the HDX transponder's identification code. The term “half-duplex” derives from the HDX transponder's behavior wherein the exchange of the activation signal and the identification code signal occur during alternate time intervals.
FIG. 2 provides a block diagram illustration of an HDX identification tag. Referring to both the block diagram of FIG. 2 and the HDX waveforms of FIG. 3(c), an HDX tag receives an activation signal from the interrogator, which manifests as a 134.2 KHz sinusoidal voltage FO illustrated in FIG. 3(a) appearing across the terminals 205a, 205b of the Resonant Antenna Circuit 201. This voltage 224 is converted to direct current and powers a portion of tag circuitry 222 that controls the accumulation of electrical charge in a capacitor 220 and also holds the tag in a suspended communication state. When the HDX tag power control circuitry 222 has detected that the sinusoidal voltage FO has diminished in amplitude, the HDX tag enters its transmission active state by supplying voltage 223 to internal circuitry.
The Clock Generator 206 in the HDX tag, in conjunction with the Resonant Antenna Circuit 201 includes a ringing oscillator, that continues to oscillate at its natural frequency, (which is approximately the same as the activation signal frequency FO), when FO 204 ceases. A ringing oscillator operates in a manner very much like a musical instrument's string, which is periodically plucked so that is remains oscillating. Such a ringing oscillator is disclosed in U.S. Pat. No. 3,995,234, the disclosure of which is incorporated herein by reference in its entirety. The oscillator output F1 207 is applied to the frequency divider which reduces F1 207 by a factor of 16, which in turn becomes signal FBR 209 having approximate frequency 8387 Hz. This frequency establishes the bit rate of the tag, and it is used to clock Binary Data 211 out of the ID Code Memory 210, wherein the Binary Data 211 resides as a sequence of binary 1's and 0's (see for example FIGS. 3(b1), 3(b2)). In other words, for every 16 input pulses of F1 207, a new identification code Binary Data bit is output from the ID Code Memory 210.
Binary 1's and 0's, such as the NRZ binary data illustrated in FIGS. 3(b1), 3(b2), are clocked out of the ID Code Memory 210 so that Binary Data 211, is applied directly to Modulation Switch SM 215. Switch SM 215 opens and closes in response to the binary 0's and 1's, respectively. In an HDX transponder, Load Impedance ZM 216 is typically a capacitive element that is connected across the Resonant Antenna 201 when switch SM 215 closes in response to a binary 1. This capacitor ZM 216 has the effect of altering the effective resonant frequency of the Resonant Antenna 201 thereby altering the operating frequency of the ringing oscillator to 124.2 KHz. Consequently, the oscillator output F1 207 becomes 124.2 KHz, which is reduced by a factor of 16 by the Frequency Divider 208 to produce the signal FBR 209 having the approximate frequency 7762 Hz. The ringing oscillator changes its frequency between 134.2 KHz and 124.2 KHz in response to binary 0's and 1's, thus creating a frequency shift keyed (FSK) sinusoidal signal (see for example FIG. 3c) that appears across the resonant antenna circuit 201. As can be seen from FIG. 3c, the period of a binary 1 is greater (about 129 μsec) than the period of a binary 0 (about 119 μsec), since the bit rate is determined by dividing the ringing oscillator's instantaneous frequency (either 134.2 KHz or 124.2 KHz) by 16.
FIG. 4 illustrates the spectra for the HDX tag, where the activation signal 401 appears at 134.2 KHz, and where the HDX transponder frequencies appear at 124.2 KHz 402a and 134.2 KHz 402b. Since the activation signal 401 and the HDX transponder signals 402a, and 402b alternate in time, the 134.2 KHz activation signal 401 and the 134.2 KHz transponder signal 402a, and 402b do not occur simultaneously. Thus, the interrogator's receive circuitry is able to detect the transponder data signal without being interfered with by its own activation signal. The frequency response 403 of a resonant antenna configured to detect FSK data transmitted by an HDX tag is also illustrated in FIG. 4.
Previous implementations of HDX receive circuits have used components that were manually tuned. For example, a typical FSK receiver down converts the received data signal to an intermediate frequency by mixing the received data signal using a mixer tuned to a frequency slightly above or below the carrier frequency. An RFID reader 500 that utilizes an architecture similar to that of a superheterodyne receiver, involving the use of a local oscillator to down-mix an FSK modulated HDX signal to an intermediate frequency is illustrated in FIG. 5. An FSK signal modulated onto a carrier 501 is received via a resonant antenna 502 and is provided to an RF amplifier 505 before mixing the received signal to an intermediate frequency using an IF amplifier 504 and a local oscillator 505. The data is then demodulated using a demodulator 506 and the demodulated data 507 is output. Tuning of the local oscillator can be critical to the operation of the RFID reader circuit illustrated in FIG. 5 as shifts in the frequency of the local oscillator relative to the frequency of the carrier can result in the reversal of the logic levels in the demodulated data depending upon whether the local oscillator is tuned to a frequency above or below the carrier frequency.
Previous implementations have also commonly used quadrature demodulators that create an analog output level proportional to frequency. This type of circuit uses a manually tunable inductor to set the centre frequency. Additional circuitry such as an Analog to Digital (A/D) converter or a comparator can be used to extract modulation data. An RFID reader 600 including a quadrature demodulator is shown in FIG. 6. The HDX FSK signal 601 is received using a resonant antenna 604 including an inductive 602 and a capacitive 603 component. The received signal is provided to a tuned radio frequency amplifier 605 and then to a quadrature demodulator 604, which outputs a pulse width modulated signal as a function of the FSK frequency. The output of the quadrature demodulator is provided to a filter network 607 that averages the pulse width variations such that the two FSK frequencies produce different voltage levels. A comparator 606 compares the output of the filter network 604 to a threshold voltage 610 to produce an output data signal 608, which is a binary signal. While FIG. 6 shows a quarature demodulator 606 used for FSK demodulation, alternate methods involving a phase locked loop (PLL), ratio detector, discriminator, and/or a pulse counter can also be utilized in the demodulation of FSK signals.
While the TRF receiver architecture illustrated in FIG. 6 is effective and economical, its principal disadvantage is its poor selectivity. Its bandwidth and susceptibility to radio frequency signals is limited by the characteristics of the resonant antenna circuit, and by the bandwidth of the TRF amplifier. The TRF amplifier may have one or two filter networks including fixed value or adjustable components, that provide modest rejection of nearby radio frequency interference signals. However, RF signals from electronic devices such as variable speed motor drive controllers, power inverters, LCD screen backlights, and proportional cycle AC controllers can introduce disruptive interference, despite being outside the HDX FSK bandwidth, because the TRF receiver lacks sufficient signal selectivity (or sufficient out-of-band signal rejection).
While it is technically possible to improve the TRF receiver's selectivity by increasing the complexity and sophistication of filters associated with the TRF amplifier, in practice this becomes expensive, difficult, and unreliable. Such filters typically require precision tuning and alignment at the point of manufacture, and thereafter would be susceptible to drift due to aging and environmental influences. Therefore, what is needed is an improved receiver design that possesses improved selectivity, while being economical and having long-term stability and reliability.